The cast molding process offers a low cost method for manufacturing components having a high dimensional figure of merit and excellent surface finish. For this reason, the cast molding process is the process of choice when manufacturing optics. Both whole lenses and individual surfaces may be cast molded. Generally, it is advantageous to mold surfaces that have to be replicated many times, while it is more cost effective to use machining techniques to produce surface geometries which are unique to a particular application, e.g., geometries required by an individual ophthalmic prescription. For this reason, whole ophthalmic lenses as well as semi-finished ophthalmic blanks have been manufactured by cast molding processes over the last thirty years.
In cast molding ophthalmic optics, the resin is selected so that it has good adhesion to the mold surface in the uncured or semicured state. This requirement ensures that the mold is pulled inward as the resin undergoes shrinkage during the curing process. If a gasket is used to secure a pair of molds to form the cavity enclosing the resin, the compressive force exerted by the molds in contact with the resin under cure causes deformation and shrinkage of the gasket. If a gasket-free molding process is used to cast mold the optic, such as described in U.S. Pat. No. 4,623,496, issued to J. Verhoeven, and in U.S. Pat. No. 5,178,800, the mold moves in response to shrinkage of the resin, and develops the final optical geometry (resin thickness from point to point). Lacking good adhesion to the mold or molds, the resin pulls away from the mold surface during the molding process, causing a defect to form at that particular point. Therefore, good adhesion between the mold and the resin is a prerequisite for successfully molding high fidelity optics. Unfortunately, good adhesion between the mold or molds and the resin also makes it difficult to demold the optic at the end of the cure process. Too strong an adhesion between the mold and the resin causes the mold or the optic to crack, or the mold to suffer "pull out" during the demolding process.
To balance these competing factors, it has been necessary to develop an optimum level of adhesion between the resin and the mold surface. Mold release agents may be added to the formulation to reduce adhesion between the cured resin surface and the mold without significantly reducing the adhesion in the precured or semicured state. Even then, demolding the optic upon completion of the cure cycle requires that a considerable force or mechanical impact be applied, thus shortening mold life. In certain instances, air hammers, mechanical wedges, and conventional hammers have been employed to open molds.
It would, therefore, be desirable to provide a method for demolding optics which eliminates or minimizes the mechanical force which is otherwise required to separate the mold from the optic.